On a recent stroll down the fifth-floor cell block of his urban jail, Sheriff Richard Stanek of Minneapolis glanced through door windows at inmates and lamented shortcomings of the nation's mental health care system.

"If you look just in this jail, with 38,000 inmates who come through here every year, about one third of them suffer from some form of severe or untreated mental illness," Stanek said.

While most are non-violent, he added, "untreated, we have a problem."

Eight of the nine killers in mass shootings last year had histories of mental illness. Few are believed to have been under the care of a mental health care provider or part of an organized community support system. None were blocked from legally purchasing a weapon.

"We have an access problem," Stanek said. "Those with severe mental illness should never get access to firearms."

As lawmakers search for solutions on gun violence in the wake of the Sandy Hook Elementary School shooting in December, there is growing consensus on the need for more resources to identify and treat people who are mentally ill -- and implement greater protections to keep dangerous people away from guns.

A bipartisan group of senators, including Republican Sen. Marco Rubio of Florida and Democratic Sen. Debbie Stabenow of Michigan, on Thursday proposed spending $1 billion over the next decade to strengthen the mental health care system.

The plan would expand grants to community health centers and encourage better integration of mental health services into existing programs. President Obama and House Democrats have backed similar measures.

Stanek, a Republican former state representative who opposes new gun controls, is also on board, emerging as a top advocate for more mental health resources in his role as president of the Major Counties Sheriffs' Association and using meetings at the White House to rally support.

When Obama visited Minneapolis this week, Stanek was by his side, continuing to emphasize mental health and background checks rather than a ban on assault weapons or high-capacity ammunition clips in the effort to curb gun violence. The bans, he says, would have done little to stop mass shooters of the past year.

"When we run the background check for you to purchase or acquire a handgun, if we say, 'It's OK, nothing comes back,' you should be good to go. But the fact of the matter is, it's not," he said. "And it's really become ... America's dirty little secret."

Just 12 states actively submit mental health records to the federal background check system, according to a recent Government Accountability Office report. Those that do are only required to provide documents related to court-ordered committals.

Thousands of felony convictions across the country have also been left out of the system, advocates say.

In many states, the records are still paper-based and have to be entered into the background check system by hand -- a costly and time-consuming process.

Adam Lanza, the 20-year-old who charged into the Newtown, Conn., school, murdering 20 children, six adults and killing himself, was known to be mentally unstable.

Three months earlier, in Minneapolis, another young man with a history of mental illness -- 36-year-old Andrew Engeldinger -- shot six at a suburban sign company before turning the gun on himself. It was the worst workplace shooting in the state's history.

Neither man had a criminal record nor were they undergoing treatment for mental illness, much less determined by a court to be dangerous. They easily cleared background checks to purchase a gun.